1. Technical Field
The present invention relates to a quantitative analysis method for measuring a target element in a specimen using a laser-induced plasma spectrum. More particularly, the present invention relates to a method for analyzing a composition ratio of a target element by calculating peak intensities when peaks overlap each other in a spectrum, and a method for selecting a peak of a wavelength at which the highest precision and reproducibility are secured when an internal standard method is used for quantitative analysis of the target element.
2. Description of the Related Art
Since laser-induced plasma emits light of a specific wavelength depending on materials, components of a material can be qualitatively or quantitatively analyzed by collecting the light. Laser-induced breakdown (plasma) spectroscopy (LIBS or LIPS) is one method for analyzing components of a material using collected light based on spectroscopic analysis using plasma, created by inducing breakdown which is a discharge phenomenon using a high power laser, as an excitation source. In plasma induced by a laser, a specimen is vaporized to generate atoms and ions present in an excited state. The atoms and ions in the excited state return to a ground state after a certain lifetime by emitting electromagnetic waves at an intrinsic wavelength depending on the types of elements and excited states. Thus, by analyzing the emitted spectrum of the wavelength, the components of the material can be qualitatively or quantitatively analyzed.
FIG. 1 illustrates an operation principle of LIBS using a typical technique.
Referring to FIG. 1, in step 102, when a very small amount (a few μg) of a material is ablated (in which a material is removed through vaporization by a laser beam) upon irradiation of a pulsed laser beam, the ablated material is ionized by absorbing laser energy within an extremely short time (generally, within a few nanoseconds), and then, high-temperature plasma of about 15,000K or more is created in step 104. After irradiation of the pulsed laser beam, each element present in the plasma emits a specific spectrum corresponding thereto while the high-temperature plasma is cooled. Then, the spectrum is collected and analyzed using a spectrum analyzer in step 106, and an intrinsic spectroscopic data of each element is obtained in step 108, so that the composition and amounts of the components of the material can be measured through analysis of these data.
Such an LIBS technique is different from other measurement techniques in terms of: 1) Time for entire measurement is within one second; 2) There is no need for separate sampling and preprocessing for measurement; 3) Since only an infinitesimal amount (a few μg) of a material is used for each measurement, a composition of the material can be measured with nanometer-level precision while the material is ablated in a depth direction; 4) There is no need for separate conditions for measurement and the measurement can be carried out in air; 5) All elements except for inert gas can be analyzed with ppm-level precision; and 6) equipment can be prepared at relatively low costs.
In spectrum analysis by typical LIBS, the intensity of the spectrum is calculated by integrating the spectral line from the baseline. When peaks adjoin each other, peak intensity can be calculated by a curve fitting method. However, when the peaks are too close to each other, there is a problem in that accuracy of a calculated value is deteriorated.
Moreover, in an internal standard method by which components of a specimen to be tested are quantified from the ratio of peak intensities of different elements of the specimen, peaks are typically selected using a limit of detection, a relative standard deviation, and the like. However, the limit of detection is a concept for a material having an extremely low concentration, and when the relative standard deviation is applied to a nonlinear calibration curve (nonlinear calibration curve appears in analysis of an element having a high concentration), an optimal peak cannot be selected.
Therefore, there is a need for a method, by which intensity of the spectrum can be measured even when peaks overlap each other in a spectrum using LIBS, and which can select an optimal spectrum in use of an internal standard method.